Coiled tubing injectors like that shown in FIG. 1 were originally used during workover operations to inject a relatively small diameter, continuous length of coiled tubing into a well bore while the well was under pressure. Coil tubing has been used for many years in wells for performing certain downhole operations such as, washing out sand bridges, circulating treating fluids, setting downhole tools, cleaning the internal walls of well pipes, conducting producing fluids or lift gas, and a number of other similar remedial or production operations. In such a case, the tubing must be literally forced or "injected" into the well through a sliding seal to overcome the well pressure until the weight of the tubing exceeds the force produced by the pressure acting against the cross-sectional area of the tubing. Thereafter, the weight of the tubing has to be supported by the injector. The process is reversed as the tubing is removed from the well. In recent years, the coiled tubing has been used in combination with a mud turbine motor to drill original bores, has been used as the permanent tubing in production wells, and continues to be used in various workover and service applications. Because of the advantages of continuous coiled tubing, and the resulting new uses, the state of the art of manufacturing coiled tubing has rapidly progressed until tubing is almost three inches in diameter. These large tubings have a wall thickness and sufficient tinsel strength to support up to 20,000 feet hanging in a well bore.
The only method by which a continuous length of tubing can be either forced against pressure into the well, or supported while hanging in the well bore, while lowered or raised is by continuously gripping the tubing along its length. One way of achieving this is by utilizing a pair of opposed endless drive chains which are arranged in a common plane. Such drive chains are made up of links, rollers and gripper blocks. In some cases gripper inserts are used. These drive chains are generally driven by sprockets powered by a motor, such as a reversible hydraulic motor. The opposed drive chains grip the reeled tubing between them. These drive chains are backed up so that a number of pairs of opposed gripping blocks are in gripping engagement with the tubing at any given moment. As the chains are in motion and the tubing is being driven, each time a pair of gripper blocks is actuated to release their hold on the tubing another pair is actuated to gripping position. The moving drive chains are thus able to force the tubing into the well, or to remove the same therefrom depending upon the direction in which they are driven.
In order to handle progressively larger, longer, and heavier tubing, the gripping force must be progressively increased. This can be achieved by increasing the force pressing the gripper shoes against the tubing, by increasing the number of gripper shoes by increasing the length of the chains, by increasing the contact area of the gripper shoe, or by improving the gripping surfaces.
As the length of the chain increases, the tolerance problems to insure that all of the individual grippers are contacting the tubing with equal force presents a practical limitation, which has been overcome in current designs. The application of greater force to press the grippers against the tubing, however tends to deform the tubing. Current grippers are typically surfaced with carbide grit which can penetrate the surface of the tubing to the point of damaging the tubing. The use of carbide surface treatment is relatively extensive and the expensive chain must be replaced frequently because when the grit wears smooth, the grippers can no longer effectively handle the tubing. The grippers can encircle only a limited percentage of the circumference of the tubing because the grippers must engage and disengage from the tubing at the beginning and end of the active reaches of the chains, and any attempts to increase the circumferential contact has resulted in unacceptable marring of the surface of the tubing to the point of causing the tubing to fail. Any increase in the force applied to the gripper results in unsatisfactory deformation of the tubing, typically causing it to become permanently egg-shaped. All of these design variables have reached the practical limits using current designs for moderately sized tubing, and the larger tubing cannot be satisfactorily handled, except in relatively shorter lengths, with even larger and longer tubing presently being demonstrated.
The coil tubing has developed high performance, advanced composite tubing. One example of such composites is epoxy resins reinforced with fibers such as carbon or glass. Composite tubing is highly desirable because it possesses many characteristics that are superior to steel, such as, good corrosion resistance, high strength to weight ratios, low material density, good low cycle fatigue resistance and improved working pressures. However, existing coil tubing injectors are not designed to accommodate the outer surface of composite tubing and tend to mar and tear the tubing, reducing its useful life.
U.S. Pat. No. 3,754,474 issued to Alexander Palynchuk on Aug. 28, 1973 for "Gripper Pad". This patent discloses a gripper unit for use in an apparatus such as that disclosed in U.S. Pat. No. 3,559,905. This gripper unit includes "a gripper pad which includes a block having studs embedded therein. The block is made of a deformable elastomer material and studs are made of a metal softer than steel, preferably aluminum or aluminum alloy. With this design, the studs come into contact with the tubing to be injected. U.S. Pat. No. 3,754,474 is incorporated into this application by reference for all purposes.
U.S. Pat. No. 5,094,340 issued to Avakov on Mar. 10, 1992 for Gripper Blocks for Reeled Tubing Injectors discloses a gripper block having a V-shaped gripping surface that forms ridges. The gripping surface contacts the tubing at four locations spaced at 90 degrees around the tubing. U.S. Pat. No. 5,094,340 is incorporated into this application by reference for all purposes.
U. S. Pat. No. 5,188,174 issued to Anderson on Feb. 23, 1993 discloses a tubing injection apparatus with light-weight gripper blocks that are pressed against the tubing with varying hydraulic force. U.S. Pat. No. 5,188,174 is incorporated into this application by reference for all purposes.
None of the prior art teaches or suggests gripper blocks for reeled or coil tubing which will provide an adequate grip for tubing or similar objects of various sizes made from fragile composite materials currently in use and actively being developed for the coil tubing industry.
Grippers for reeled or coil tubing generally have been formed from steel and provided with notches having a radius slightly greater than that of the tubing. The steel grippers took a vise-like grip on the tubing and although they fit the tubing fairly closely, excessive squeeze often resulted in the tubing being distorted to an out-of-round condition and scarred.
Such gripper blocks are used extensively today, and since it is known that they can distort and scar the tubing it is desirable to provide improved grippers for reeled tubing which will overcome the shortcomings of the existing grippers. The present invention is an improvement over the gripping blocks used in the known prior art and overcomes many of the shortcomings associated therewith, and are more suitable for use in the modern oil industry where wells are deeper, conditions more severe, operations more costly, and damaged or ruined reeled tubing can cause considerable delays and added costs.